Shell and tube heat exchangers have an array of tubes extending between and through two spaced apart tube sheets surrounded by a shell. The shell is provided with an inlet and an outlet so that a suitable heat exchange liquid or gas can be circulated through the shell to cool or heat a liquid flowing through each tube.
Each end of the array of tubes can be left open, or exposed, for use in some processing operations. For other operations, one or both ends can be enclosed by a liquid retaining header, which may or may not have a removable cover or access port. When only one liquid header is present, it can be either a liquid inlet or liquid outlet. When a liquid header is positioned at each end, one liquid header can constitute a liquid inlet while the other can be a liquid outlet. Such an arrangement is conventional for once-through or single pass heat exchangers. The liquid inlet and outlet headers, or portions thereof, are provided with suitable conduit means for supplying and removing liquid.
Although shell and tube heat exchangers are generally used to heat a liquid feed stream, they can be used for cooling such a stream. Shell and tube heat exchangers of the described types can be used as freeze exchangers for producing fresh water from brackish water and seawater, for concentrating fruit and vegetable juices, and in industrial crystallization processes. As the liquid flows through each tube, it can be cooled enough to crystallize a solid from the liquid. Thus, by cooling seawater, ice is obtained which when separated, washed and melted provides potable water. When a fruit or vegetable juice is similarly chilled, ice forms and is removed to provide a concentrated juice.
Heat exchangers of the described types can use any cooling fluid on the shell side to cool a liquid flowing through the tubes. The fluid can be fed through one end and removed through the other end of the heat exchanger in a substantially unidirectional flow. Some suitable cooling fluids are ammonia and Freon brand refrigerants.
To obtain optimum heat exchange it is desirable in many instances for the tubes to be arranged vertically and for the feed liquid to be supplied to the tube surfaces as a downwardly flowing or falling liquid film. Not only is the feed liquid brought more quickly close to the temperature of the heat exchange liquid in this way but less recirculation of the liquids is required, thus reducing energy consumption.
It is customary to locate heat exchange tubes close together and parallel to one another and often with a tube pitch of 1.25 times the tube diameter. Tube pitch is the distance between the center of two adjacent tubes in any direction. One of the most efficient ways to seal the tubes to the tubesheets is to roll expand the tube against the wall of the hole in the tubesheet through which the tube extends. In this way a liquid tight seal is readily and inexpensively produced. Formation of joints by roll expansion, however, usually have a crevice or gap in the space between the tube wall and the surfaces of the tubesheets. This crevice may extend inwardly from the tubesheet surface about 0.25 to 1 inch depending somewhat on the thickness of the tubesheet and the roll forming technique.
In many cases the existence of the crevices creates no problem. However, when some liquids, such as those which are corrosive, are treated in a heat exchanger the crevices have been found to be a prime location of corrosion, even when stainless steel is used for the tubes and tubesheets. Such corrosion is not restricted to elevated temperatures but also occurs in freeze exchangers as, for example, when fresh water is produced from salt water and brackish water by production of ice and subsequent melting of the ice. Furthermore, when the tubes are simply rolled into a tubesheet it is very likely that a leak may develop at those joints thus leaking the refrigerant into the water box. This can occur even if the liquid flowing through the tubes is not corrosive.
Not only is it desirable to provide some means to eliminate crevice corrosion and leaks but the solution to those problems should permit fabrication of a heat exchanger with the upper ends of the tubes extending above the top surface of the upper tubesheet. This permits the feed liquid to flow around the tubes and form a pool before it spills over the lip of the tubes as a falling film, thereby achieving a more even distribution of liquid to the tubes.